1. Field of the Invention
This application relates to magnetic resonance imaging.
2. Description of the Related Art
Diffusion-weighted magnetic resonance (MR) imaging (DWI) is a known tool for detecting abnormal water diffusion in the brain (e.g., ischemic stroke). The directional information obtained using diffusion tensor MRI (DTI) is valuable in understanding as well as evaluating white matter abnormalities in neurological diseases, such as Alzheimer disease, schizophrenia, multiple sclerosis, and neurofibromatosis. DWI and DTI may also give useful information about the development and disorders of ordered structures in extracranial organs such as the heart, kidney, breast, and prostate.
Although DTI can provide useful information about white-matter diseases in the brain, high resolution DTI of brain regions near the temporal bone or sinuses, of small neural structures such as the spinal cord or optic nerve, or of extracranial organs in vivo has been difficult to achieve using conventional two-dimensional (2D) singleshot diffusion-weighted EPI techniques (2D ss-DWEPI). There are strong non-uniform local magnetic fields created by the magnetic susceptibility changes at tissue/bone or tissue/air interfaces, which typically induce severe distortion on the resultant ss-DWEPI images. The amount of susceptibility induced geometric distortion is proportional to the total sampling time in EPI. Typically, increasing spatial resolution requires an increase in the duration of the data acquisition window, which in turn increases the distortion from off-resonance effects. As a result, the spatial resolution obtained using conventional 2D ss-EPI is generally much lower than that obtainable with conventional, multi-shot MRI, giving decreased resolution for measurements of interest, such as white matter tract anatomy and nerve fiber anatomy. For these reasons, 2D ss-DWEPI has been clinically useful only for moderately low resolution intracranial applications. EPI with parallel imaging has been successfully applied to high-resolution brain DWI and DTI studies resulting in substantial image quality improvement.
There are several non-EPI singleshot DWI techniques, which include multiple spin-echo sequences (e.g., ss-FSE (or HASTE) and GRASE), STEAM, and fast gradient echo sequences (FLASH), that complete the total data acquisition following a single diffusion weighting. These 2D sequences typically acquire slightly more than half of the ky encodings in about 500 ms after a single diffusion weighting preparation. These non-EPI singleshot techniques typically employ relatively thick slices to overcome their intrinsic low SNR.
Multishot imaging techniques may be used to increase SNR, improve spatial resolution and reduce susceptibility induced artifacts. However most multishot DWI acquisition techniques suffer from the instability of phase errors among shots due to global or localized motions during application of the large diffusion gradients. There has been reasonable success with techniques that use navigator echoes to detect and correct phase errors, or that use non-singleshot-EPI approaches that are less sensitive to phase errors. Because most of these are 2D acquisition techniques, they produce relatively poor resolution along the slice direction.